<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

    </style>
</head>
    <div>
        <span class="hour">1</span>
        <span class="minute">2</span>
        <span class="second">3</span>
    </div>
<body>
    <script>
        //1.获取元素
        var hour = document.querySelector(".hour");//小时的盒子
        var minute = document.querySelector(".minute");//分钟的盒子
        var second = document.querySelector(".second");//秒数的盒子
        var inputTime = new Date("2022-7-5 11:00:00");//返回的是用户输入时间总的毫秒数
        countDown();
        //2.开启定时器 
        setInterval(countDown, 1000);
        function countDown() {
            var nowTime = new Date(); //返回的是当前时间总的毫秒数
            var times = (inputTime - nowTime) / 1000; //times是剩余时间总的秒数
            var h = parseInt(times / 60 / 60 % 24);//时
            h = h < 10 ? 0 + h : h;
            /* if(h < 10) {
                h = '0' + h
            } else {
                h = h
            } */
            hour.innerHTML = h;//把剩余的小时给 小时
            var m = parseInt(times / 60 % 60)//分
            m = m < 10 ? '0' + m : m;
            minute.innerHTML = m;
            var s = parseInt(times % 60);//当前的秒数
            s = s < 10 ? '0' + s : s;
            second.innerHTML = s;
        }
    </script>
</body>

</html>